Results of a recent study at the Pennington Biomedical Research Center in Louisiana indicate that excess calories, whether from low- or high-protein diets contribute to weight gain and body fat. However, protein levels appear to influence how the body metabolizes those calories, with people eating high-protein diets gaining more lean body mass and expending more energy.

The study was reported in the Journal of the American Medical Association.

The researchers recruited 25 people who lived as in-patients for 56 days, where they overate by about 1,000 calories per day. During the “overeating” period, the researchers fed one group a diet containing just 5 percent protein, while a second group ate more normal diet containing 15 percent protein and a third group consumed a high-protein diet containing 25 percent protein.

As you might expect, all the groups gained weight and gained body fat. On average, the low-protein group actually gained the least weight, at 3.16 kilograms, while weight gains averaged 6.05 kilos in the normal protein group and 6.51 kilos in the high-protein group. However, in the low-protein group, virtually all the gain was body fat.

Total body-fat gain over the test period was about the same for each group, averaging 3.51 kilos. The remaining weight gain in the normal and high-protein groups came from lean body mass. The low-protein group, on average, lost 0.7 kilos of lean body mass during the test period while they put on fat. The normal-protein group averaged a gain of 2.87 kilos of lean body mass and the high-protein group gained an average of 3.18 kilos of lean body mass.

“The failure to increase lean body mass in the low protein group accounted for their smaller weight gain,” the researchers wrote.

Part of the reason for these differences could be that in this study, overeating led to a significant increase in resting energy expenditure in both the normal and high protein groups, but resting energy expenditure in the low protein group did not change significantly, and was significantly lower during the eight weeks of overeating than the other two groups.

The researchers concluded that while weight gain when eating a low-protein diet was lower compared with higher-protein diets with the same number of extra calories, calories alone contributed to the increase in body fat. In contrast, protein contributed to the changes in energy expenditure and lean body mass, but not to the increase in body fat.

My own interpretation, which is outside the parameters of this study and the researchers’ conclusions, is this: If the same patterns hold true at normal, healthy calorie-intake levels, perhaps a normal- to high-protein diet produces more lean body mass, higher metabolism and less body fat compared with diets deficient in protein.

An abstract of the study and the full report are available online from the Journal of the American Medical Association.